Nowadays, in the field of optical fiber communication, there are increasing demands for large capacity of information to be transmitted and long transmission distance. Hitherto, the optical communication system, in which a single optical signal is transmitted through a single optical fiber in an optical cable, has been put to practical use. However, in the aforementioned optical communication system, a transmitter and a receiver are required to have wide band characteristics in order to increase the capacity of information to be transmitted. Moreover, an average received power level must be increased in order to decrease error rate of a received signal, and a spacing between adjacent repeaters must be decreased in the case of a multi-stage repeater system.
On the other hand, the development of the WDM optical transmission system, in which the plural optical signals with the different wavelengths are multiplexed and propagate through the same optical fiber, is being actively developed as a technology for increasing multiplexed density of the optical fiber communication without increasing the transmission capacity of a single optical signal.
In the WDM optical transmission system, the optical signals are transmitted through the optical fiber and amplified by a multi-stage repeater system, which is arranged along the optical fiber line. In many cases, Er-doped optical fiber amplifiers are used as the optical direct amplification repeaters. However, since the gain of the Er-doped optical fiber amplifier depends on the wavelength of the optical signal, a characteristic dependence of the gain on the signal wavelength is accumulated by the multi-stage amplifications, and the gains of the multiplexed optical signals are changed in accordance with their wavelengths. Consequently, the optical signals corresponding to smaller gain are attenuated in the multi-stage amplifications, and it has been an important factor of difficulty of a long distance transmission.